Bone screws and pedicle screws are known from the state of the art. They serve, for example, for the dorsal stabilization of the spinal column by means of transpedicular screwing. Pedicles screws are placed in the pedicles of adjacent vertebrae, whereupon an angularly stable connection is made between the pedicle screws which are axially superimposed and an axially extending longitudinal support or land. The pedicle screws and the longitudinal supports form a vertebral stabilizing system.
Usually a pedicle screw includes a screw shank extending in the axial direction and including an external thread to which a receiving sleeve, the so called tulip, is connected on the side of the screw head. Said tulip is substantially U-shaped having opposed wall portions and a gap formed therebetween and extending in the radial direction for the longitudinal support or land. An internal thread extending in the axial direction is introduced into the tulip. The longitudinal support is inserted in the gap of the tulip in the radial direction and is fixed by means of a locking element or a clamp screw, typically in the form of a stud screw or threaded nut, which is also referred to as set screw and is screwed into the internal thread.
When a set screw is attached to a pedicle screw, especially in the case of an open operation there is a risk of jamming due to tilted attachment of the set screw. This situation occurs in particular when the thread of the set screw and the internal thread of the tulip are twisted about approx. 180° when the set screw is attached, i.e. a threaded flank abuts a threaded flank, as illustrated in the enclosed FIG. 1.
In the case of known pedicle screws, even when the set screw is correctly attached, slight tilting or jamming of the set screw relative to the axis of the pedicle screw and, resp., the tulip thereof may entail a “rough” attachment, which increases the risk of jamming. To make matters worse, in practical use the set screw is usually pressed onto the tulip with a certain force. Said axial pre-force may detrimentally result already in the set screw getting jammed with respect to the tulip, as the planar upper side of the tulip usually does not correspond to the thread geometry of the set screw.
Jamming may lead to the thread chamfer, especially the one of the set screw, being damaged. In the worst case, a so called “cross-threading” may occur, meaning that the set screw is tilted with respect to the longitudinal axis of the screw shank and the external thread thereof so far that the thread start of the set screw, viz. the lead-in thread turn or turns, engage(s) in the wrong thread turn of the internal thread of the tulip, which may lead to damage of the thread up to uselessness of the set screw and/or of the pedicle screw. A clicking when attaching the set screw which is caused by jamming will unsettle the user. The latter is not sure whether the thread has been damaged.
In order to reduce the afore-described problems, particular thread designs such as e.g. rectangular threads or undercut threads are known and are described to “reduce cross-threading”. However, correct attachment of the set screw appears to be the crucial action for avoiding damage of threads. For this purpose, pedicle screw systems are known in which a rotatable guide cap is mounted on the set screw and enables the set screw to be additionally guided in the tulip head thread. It is moreover known to chamfer the thread lead-in of the set screw, in other words to reduce the thickness thereof. In this way, engagement of the thread lead-in of the set screw into the thread lead-in of the tulip head is facilitated. Apart from that, there are systems offering a guide for the set screw so as to prevent the same from tilting with respect to the screw axis. This guide subsequently will be broken away. During percutaneous operation the coupled sleeve may prevent the set screw from tilting in numerous cases. During open operation a set screw starter instrument including a guiding sleeve aligning relative to the body may limit such problematic tilting.
The afore-described pedicle screw systems known from the state of the art are disadvantageously cost-intensive and complex. As a rule, they are not adapted to facilitate and reliably ensure correct attachment of a set screw to a pedicle screw. A screw driver including a guiding sleeve may prevent tilting to a restricted extent only, as said guiding sleeve requires a certain play for coupling. In addition, said sleeve restricts the surgeon's vision onto the bone or pedicle screw. An optimization of the chamfer improves finding the thread, but jamming may occur despite this measure. Jamming of a “chamfered” set screw is critical, as the latter may be damaged very easily due to the smaller wall thickness.